Depending how much crushed coral you have mixed in and how coarse the sand is will make a big difference, If it not fine enough then you won't have an anaerobic area needed to process the nitrates. DSB required fine sand to function, I don't see where this would be any different.
This may or may not be your problem.
EDIT: I tried finding an old article by R Shimek but had no luck, In the article he talk about the size of the substrate needed for a deep sand bed to function, Anyone else know where to find that information?
Quote:
Originally posted by EnderG60
its pretty fine, not quite sugar fine, with some crushed shells mixed in
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